Flavins are versatile redox cofactors which are used in enzymes to perform a wide range of vital chemistry ranging from fatty acid metabolism to reductive detoxification of Hg 2+. This is a reflection of the flavin's electronic flexibility and amenability to tuning via a variety of interactions at multiple functional positions. We seek to understand individual mechanisms by which proteins specify the redox activity of flavins, beginning here with hydrogen bonds and bending of the flavin isoalloxazine ring system in simple models and two complementary proteins. We propose a description of enzymatic flavin redox tuning at the level of electronic structure using a combination of (1) the first use of solid state NMR to measure the principal values,delta/ii, of the chemical shift tensors of flavin carbons and nitrogens as sensitive and direct reflections of flavin electronics and interactions, (2) computation of electronic structure, energies and chemical shifts in order to permit quantitative interpretation of the observed chemical shifts in terms of flavin electronics and interactions, (3) NMR studies directly evaluating the importance of hydrogen bonding in redox tuning in nitroreductase and (4) mutagenesis of flavodoxin and nitroreductase to manipulate flavin bending, a controversial potential mechanism of flavin redox tuning. Thus, we will identify 15N and 13C signatures of flavin bending and the different hydrogen bonds the flavin makes, and begin to understand at a fundamental level the redox tuning applied by proteins in terms of individual interactions. These tools will have widespread applicability to flavoproteins.